Air motor driven hydraulic pumps are well known. A typical one of such pumps is disclosed, for example, in U.S. Pat. No. 4,074,612.
Most of these pumps are driven by a reciprocating air motor in which air at a pressure of about 120 psi acts on an air piston that drives a much smaller hydraulic piston to pump the hydraulic fluid. The pump is typically a single stage and operates as long as compressed air is introduced to it and it is turned on. It can be turned off, and also the treadle for operating it can be moved into a position to retract the hydraulic fluid back into the reservoir of the pump.
Such pumps are capable of generating hydraulic pressures of 10,000 psi or more, being only limited by the air pressure delivered to drive the device and by the ratio of the air piston and the hydraulic piston. At a given pressure of hydraulic fluid, these pumps would typically pump a certain flow rate, which rate went down as the hydraulic pressure went up in accordance with a certain relationship between the hydraulic pressure and the air pressure. This relationship was fixed, and was relatively continuous and gradual as the pressure changed. The result is that at high and low pressures, the performance was less than what could have been achieved with a more efficient machine.
In addition, in some applications metered delivery and metered return of fluid from the load, for example a hydraulic cylinder, is desirable, and prior pumps were lacking in this regard. Moreover, fluid contamination was sometimes an issue in these prior pumps.